For start, if you use store_script_param in your script then you're supposed to pass parameter to it while calling it
(call_script, "script_botattackbarricade", ":some_instance"), #since you passed some_instance as parameter, this would make (store_script_param, ":instance_id", 1), actually store that something into instance_id variable, whatever some_instance is, I don't see how you intend to get it from trigger
Another thing, put try_for_agents below store_script_param, as you want to call store_script_param only once, otherwise store_script_param will be performed per each agent on the map, that will cause the script to perform unneeded extra work.
Though, if I am guessing right at what you're trying to do here, you shouldn't be passing parameter and using store_script_param at all, what you want to do is loop through all the objects on the scene (there are many examples of how to do that in existing code) and then check if it is the sapper construction type, as you already do.